An electrolytic copper foil is produced by using an aqueous solution composed of sulfuric acid and copper sulfate as an electrolyte, a titanium plate coated by iridium or an oxide thereof as a dimensionally stable anode (DSA), a titanium drum as a cathode, applying a direct current between two electrodes to electrodeposit copper ions in the electrolyte on the titanium drum, and then stripping the electrolytic copper foil from the surface of the titanium drum and continuously winding for manufacturing. The side that the electrolytic copper foil contacts with the surface of the titanium drum is referred to as “shiny side (S side),” and the back side of the electrolytic copper foil is referred to as “matte side (M side).” Usually, the roughness of the S side of an electrolytic copper foil depends on the roughness of the surface of the titanium drum. Therefore, the roughness of the S side of the electrolytic copper foil is relatively consistent, whereas the roughness of the M side can be controlled by adjusting the conditions of the copper sulfate electrolyte.
The current copper sulfate electrolytes for producing electrolytic copper foils for use in lithium ion secondary batteries can be mainly classified into two major categories, and one of which is the so-called additives-containing system, i.e., to a copper sulfate electrolyte, adding organic additives such as gelatin, hydroxyethyl cellulose (HEC) or polyethylene glycol (PEG), capable of inhibiting electrodeposition of copper ions, and sulfur-containing compounds such as sodium 3-mercaptopropane sulfonate (MPS) and bis-(3-sodiumsulfopropyl disulfide (SPS), capable of refining crystalline particles. As such, the roughness of the M side of the electrolytic copper foil can be lowered, and thereby obtaining an electrolytic copper foil with double-sided gloss and having a structure containing fine crystalline particles. The other category is the so-called non-additives-containing system, i.e., no addition of any organic additives to a copper sulfate electrolyte. This type of non-additives-containing system is contrary to the additives-containing system. The lower the total content of the organics in the copper sulfate electrolyte, the higher the likelihood of obtaining a glossy electrolytic copper foil having low roughness at the M side and no abnormal protruded particles on the surface. Although no organic additives are added to the copper sulfate electrolyte obtained from the non-additives-containing system, the copper raw material used in the copper sulfate electrolyte are mainly derived from commercially available recycled copper wires. The surfaces of the copper wires contain grease or other organic substances, such that when the copper wires are dissolved in sulfuric acid, the electrolyte for producing an electrolytic copper foil would be filled with impurities like grease or organic impurities. The higher the content of the organic impurities, the higher the likelihood of generating an electrolytic copper foil having numerous abnormal protruded particles on the M side. Hence, no electrolytic copper foil having double-sided gloss is obtained.
Moreover, when the M side of an electrolytic copper foil has numerous abnormal protruded particles, the subsequent applications in the manufacture of electrolytic copper foil are usually problematic. For example, during a copper roughening treatment, the abnormal protruded particles on the M side easily induce point discharging, which cause the copper roughening particles to abnormally concentrate. Subsequently, when the copper clad laminate was formed by pressing the electrolytic copper foil, the residual copper which was formed due to incomplete etching can easily cause a short circuit. As a result, the yields of the downstream products are poor.
Due to the increasing concerns of environmental awareness, gradually single-use batteries (primary batteries) have been replaced by the high performance secondary batteries which are widely used in consumer electronic products, energy storage systems, and other industries.
Following the development of auto industry, the demands for lithium ion secondary batteries have increased. Besides the demands for favorable charging-discharging performances, the safety and battery life of lithium ion secondary batteries should also be taken into consideration. The development trend of lithium ion secondary batteries is moving toward developing power storage batteries for energy storage systems. In order to develop lithium ion secondary batteries to fulfill the system-scale requirements and satisfy the development trend of energy storage technologies, the battery capacitance of the lithium ion secondary battery has to reach the scale of MW/MWh, the cycle life of the lithium ion secondary battery used in a mobile phone should be over 2000 times and the cycle life of the lithium ion secondary battery used in an energy storage system should be over 6000 times.
A lithium ion secondary battery is produced by coiling an anode plate, a separator, and a cathode plate, placing the anode plate, separator and cathode plate into a container, adding an electrolyte and sealing the container, wherein the anode plate is consisting of a anode current collector constructed by a copper foil and an anode active material (such as a carbon-based material) which is coated on the surface of the anode current collector. The copper foil can be a rolled copper foil or an electrolytic copper foil.